1. Field of the Invention
This invention relates to a magnetic disk cartridge comprising a small-diameter, very thin flexible magnetic disk sheet housed in a case and rotated at a high speed for magnetic recording and reproduction. This invention particularly relates to a magnetic disk cartridge which prevents distortion arising at a magnetic disk sheet portion secured to a center core from adversely affecting the recording and reproduction.
2. Description of the Prior Art
Floppy disks are widely used as recording media for computers because they are easy to handle and low in cost. The floppy disk comprises a disk-like base made of a flexible polyester sheet or the like, and magnetic material layers overlaid on opposite surfaces of the disk-like base. The floppy disk is rotated at a high speed for magnetically recording information on the magnetic material layers by use of a magnetic head. On the other hand, by utilizing the aforesaid advantages of the floppy disk and the advantage that a magnetic recording medium has over silver halide photographic films in that the magnetic recording medium can be reused, it has been proposed to use a magnetic disk having a size smaller than the size of the floppy disk as an image recording medium in still cameras. The magnetic disk used for still cameras comprises a small-diameter, very thin magnetic disk sheet rotatably housed in a small case. The combination of the magnetic disk sheet with the case is called a magnetic disk cartridge.
FIGS. 4 and 5 are an exploded perspective view and a sectional side view showing the conventional magnetic disk cartridge. In the conventional magnetic disk cartridge, a hat-like center core 10 is rotatably supported in a case 11, and an inner circumferential edge porticn 12b of a magnetic disk sheet 12 having a circular hole 12a at its center is secured to a flange portion 10a of the center core 10. An end face 10b of the center core 10 is exposed as a magnetic chuck surface from an aperture 11a at the center of the case 11, and magnetically chucked to a magnetic chuck portion 13a of a rotation drive shaft 13 of the floppy disk driver. Also, engagement pins 13b and 13c of the drive shaft 13 are engaged with engagement holes 10c and 10d of the end face 10b to rotate the magnetic disk sheet 12 inside of the case 11. Normally, the inner circumferential edge portion 12b of the magnetic disk sheet 12 is secured by a ring-like double-faced adhesive member 14 to the flange portion 10a.
When the magnetic disk sheet 12 is secured to the flange portion 10a of the center core 10, wrinkle-like distortion readily arises at the portion of the magnetic disk sheet 12 secured to the center core 10, particularly in the case where a material based on a nonwoven fabric is used as the double-faced adhesive member 14. The distortion obstructs increase in the recording capacity of the magnetic disk cartridge. Specifically, in order to increase the recording capacity of the magnetic disk sheet having a defined outer diameter, the recording area of the magnetic disk sheet is expanded as close to the sheet center as possible. However, when distortion extends up to the recording area, the magnetic head of the recording and/or reproducing apparatus does not correctly contact the magnetic disk sheet. Further, since the flatness of the recording surface is low, information is not correctly recorded and reproduced.
One approach to elimination of the aforesaid problems is to make the diameter of the hat-like center core 10 small and to make small the diameter of the circular hole 12a at the center of the magnetic disk sheet 12. However, this method is not practicable since the diameter of the magnetic chuck portion 13a of the floppy disk driver and the positions of the engagement pins 13b and 13c are defined by standards.